Stress sensor

ABSTRACT

A stress or magnetic field sensor comprises a generally elongate magnetically soft amorphous or nanocrystalline electrically resistive element and biasing means for applying to the element a bias magnetic field of which the component directed along the length of the sensor has an amplitude variation pattern along the element. A periodically varying pattern has the effect of reducing the sensitivity of a stress sensor to external ambient fields (FIG.  3  shows that with a sawtooth bias field the sensitive portions a of a sensor move to positions b in the presence of an ambient field, but their number remains the same). A ramped bias field enables the position of the sensitive region of the sensor to be controlled, for measuring local stress, or for mapping an external magnetic field. Control of the regions where the sensor is active may include selective conductive coating of portions of its length. Potential uses of the stress sensor include a pressure sensor, embedment in moving parts (using rf communication) such as vehicle tyres, aircraft wings or machine parts, and in structures such as bridges where stray magnetic fields are a problem.

[0001] The present invention relates to the measurement of stress usingmagnetic stress impedance sensors.

[0002] Of the many methods for measuring stress are known in the priorart, few or none fulfil all the requirements of low cost, highrobustness and high sensitivity which are the ideal for manyapplications. Additional constraints may arise when it is also requiredthat the stress to be measured is in a moving part.

[0003] Highly sensitive sensors have been developed which employ softmagnetic materials, for example in the form of negative magnetostrictiveamorphous or nanocrystalline melt-spun wires and ribbons, and which arebased on the GMI effect.

[0004] When an ac drive current is passed through a magnetically soft(normally amorphous or nanocrystalline) electrically resistiveconductor, e.g. a wire, ribbon or fibre, the ac voltage therebydeveloped is highly sensitive to the presence or application of anexternal magnetic field, particularly when the drive current frequencyis greater than 100 kHz, the effect being known as the GiantMagneto-Impedance Effect (GMI). The change in voltage is understood asbeing a consequence of the dependence of the skin depth of the conductoron the magnetic permeability. Interpretation of the GMI effect wasintroduced in 1994 simultaneously by Panina and Mohri Appl. Phys. Lett.65 (1994) 1189 and Beach and Berlcowitz Appl. Phys. Lett. 64 (1994)3652.

[0005] Since the GMI effect can occur in long wires or ribbons, it ispossible to detect the integrated magnetic field along the path of thewire or ribbon by using the appropriate hardware, as described in ourcopending UK patent application GB 9814848.9 filed Jul. 9, 1998, andderived International Patent Application WO 99/01967 and European PatentApplication 99926653.9. The external magnetic field to be measured maybe temporally invariant, but where it varies with time it is to beexpected that the ac impedance will show a corresponding variation.

[0006] The emphasis in the aforesaid patent application is theapplication of a uniform bias field (see for example FIG. 6 of theapplication) to enable integration of the external field to be measuredalong the length of a GMI material. By contrast, as will be describedhereafter; in the sensor of the present invention a non-uniform biasfield is deliberately applied to the GMI material. The effect isdependent upon the component of the field which lies along the length ofthe sensor, and it should be understood that the non-uniformity of thebias field must correspondingly be a non-uniformity of amplitude asmeasured along the longitudinal axis of sensor.

[0007] Furthermore, because of the inverse magnetostriction effect insuch materials, the strong skin effect causes the impedance of thesensing element (the electrically resistive conductor) to change withapplied stress S, this effect being termed the Giant Stress-Impedanceeffect (GSI). The physical mechanism of the impedance change is believedto substantially avoid cross-talk problems between orthogonal componentsof the stress tensor such as can arise with conventional strain gauges,for example.

[0008] It has been found that the optimal drive frequency, i.e. thefrequency of the applied ac current, for GMI and GSI sensors lies in theMHz region, which permits relatively easy integration into an rf (radiofrequency) communication system, for example for simple interfacing witha passive rf tag system. In turn this facilitates remote sensing ofstress or related factors in moving parts. Since it has also been foundthat such sensors have only a low power requirement for satisfactoryoperation, commonly as little as a few microwatts, it is possible tolocate a sensor element within a matrix, for example of plastics orelastic material, for remote interrogation with no external leads orother attachments.

[0009] The rf system may be any known system for sensing impedancechanges in the sensor element. For example, the wire may form part of aresonant circuit which changes its resonant frequency as the impedanceof the sensor element changes. Alternatively the sensor element could beincorporated into a balance bridge providing a frequency modulatedoutput rf signal.

[0010] Furthermore, the rf system could simply be a wire itself, whichon its own can be both as sensor and antenna, as in International PatentApplication No. PCT/SE00/00476 (Tyren et al) published under serialnumber WO 00/57147. In this sensor a temporally variant rf magneticfield (referred to as a magnetic sinewave bias field when the sensor ismagnetically driven), which interacts with the magnetic moments withinthe GMI/GSI material, is used both as a magnetic excitation for thesensor and also as a communication medium in that variations in thereturn excitation signal can be measured as an indication of the acimpedance of the wire, and hence stress in the wire. While the presentinvention will require some means of sensing the impedance variation ina GMI or GSI element, and while this could be effected by any or theforegoing means, it is primarily concerned with the provision of a biasfield which varies along the sensor. As will be explained later, such abias field makes regions of the sensor more or less sensitive accordingto position.

[0011]FIG. 1 is a schematic indication as to how the complex impedance Z(consisting of reactance X and impedance R) of a sensor wire elementconsisting of a 10 cm length of (Co_(0.94)Fe_(0.06))_(72.5) Si_(12.5)B₁₅(diameter of 125 μm) alters with the applied magnetic field (H) and itwill be seen that there is a very marked increase in impedance whenevera finite magnetic field is present. It will also be seen that theresponse is independent of direction of the field along the element,that the region of greatest sensitivity (rate of change of impedancewith field) is associated with the zero field position, and that thesensitivity falls as the magnetic field increases. Hereafter this regionwill be termed the “sensitive location”. However, precisely because suchsensors are so very sensitive to magnetic fields, including straymagnetic fields that commonly occur in stress measuring environments,applications thereof have heretofore been limited or impractical.

[0012] The latter point is illustrated in FIG. 2, which shows inschematic form the characteristic variation in impedance of a GSIelement in the form of a Co-based amorphous ribbon, 20 mm long, 1 mmwide and 20 μm thick, the exact composition being unknown, under variouslevels of applied stress. While it will be appreciated that theimpedance is markedly affected by the applied stress level, particularlyat low levels of applied magnetic field, it will again be seen thatvariations in applied magnetic field also have a large influence onimpedance, thereby rendering the measurement of stress by such a sensorunreliable. It should be noted that magnetic field refers throughout tothe field component parallel to the length of the wire. Effects of thefield component perpendicular to the wire tend to be negligible due tothe large demagnetising effects in that direction.

[0013] The present invention provides a sensor comprising a generallyelongate magnetically soft amorphous or nanocrystalline electricallyresistive element and biasing means for applying to the element a biasmagnetic field, the component of said field directed along the length ofthe sensor having a spatially varying amplitude pattern along theelement. Normally the sensor element will be of an amorphous ornanocrystalline metal or alloy. The invention extends to a sensingdevice, a sensing arrangement, a method of reducing the sensitivity ofthe impedance of a stress sensor element to magnetic fields, and amethod of measuring stress in an object.

[0014] In one embodiment of the invention the bias field is arranged toreduce the effect of external ambient magnetic fields on the sensorresponse. In such a case, it is believed that the effect of applying thebias field to the sensor element is to average out the GMI/GSI responseto provide a flat, or flatter, magnetic field insensitive response.While not wishing to be bound by any theory, this is shown schematicallyin FIG. 3 where effective field H is plotted along the length L of thesensor for a bias field along the element having an amplitude whichvaries in a sawtooth manner along the element. In the absence of anyother magnetic field the sawtooth is symmetrically located about a zerofield line Ho as shown by dashed line, and sensitive locations a of thesensor occur each time the dashed line intercepts the zero field lineHo. In the presence of an additional ambient field directed along theelement the sawtooth is shifted as shown by the dotted line, and thesensitive locations are shifted along the sensor to locations b. Howeverthe number of sensitive locations remains the same, and accordingly theresponse of the sensor is insensitive to the presence of the ambientfield, relative to the case where the sawtooth bias is absent. Such anarrangement can be used to measure stress.

[0015] Care should be taken that the bias is not so strong as to makethe impedance response insensitive to stress as well as to straymagnetic fields, and in this respect it has been found that it ispossible to control the stress sensitivity and magnetic fieldsensitivity by controlling the form and intensity of the bias magneticfield applied to the sensor element. It will be understood that theoptimum form of bias field will depend on requirements for linearity ofresponse, the expected magnitude of stray fields in use for theapplication in hand, and on the manner in which impedance Z depends onmagnetic field H and stress S.

[0016] In the foregoing arrangement, in which the sensitive locations ofthe sensor vary with external ambient field but the number of sensitiveregions remains constant, the exact position of the sensitive regionstends to be immaterial. In other embodiments, however, the bias field isarranged so as to control the location of a sensitive position orpositions of the sensor. This is shown schematically in FIG. 4 where theamplitude of the bias field 1 measured along the element is ramped alongthe sensor length L to provide a single sensitive location c. In theabsence of any external ambient field, the dc component of the biasfield can be altered to displace the sensitive location c, enablingstress to be measured locally at the location c. Alternatively, using asymmetrical bias field as shown, the location c is indicative of themagnitude of any external ambient field. This may be developed into amethod of detecting or mapping ambient fields, as will be explainedlater. Clearly the bias field pattern could be such as to provide two ormore sensitive locations simultaneously.

[0017] In one preferred embodiment of sensor the amplitude of the axialcomponent of the bias magnetic field (the “pattern”) varies periodicallyalong the element. One preferred variation is sinusoidal, butalternative patterns could be used as appropriate, including sawtooth(single or double ramp) and stepped patterns, or approximations thereto.

[0018] In zero external field a sinusoidal pattern puts relatively moreof the sensor element into a high biased state (near A) than would alinear pattern (i.e. ramp, saw tooth or triangular configuration).

[0019] This means that when an external reverse field (approaching -Aalong the element) is applied to a sinusoidally biased system, the totalamount of sensor element near a net-zero-field (high sensitivity state)is higher for a sinusoidal bias field than when a linear bias field isused, resulting in a peak in the response to external field for asinusoidal bias field, rather than the flatter response which would beobtained for the linear bias field.

[0020] This is only true if the GMI response is small or relatively flatat net fields of around A, for more complex high field responses a morecomplex bias is required and preferred.

[0021] In another embodiment of the invention the bias field is notperiodic. It may, for example, take the form of a linear or non-linearramp.

[0022] The bias field (or at least the aforesaid pattern) may bepredetermined and time invariant. For example, the aforementionedsinusoidal field pattern may be applied by a magnetically loadedflexible mat, e.g. of rubber, located adjacent the sensor element, andof approximately the same length. The field pattern can be varied by theuse of specifically designed magnetiser fixtures. In one embodiment, themat was loaded with a SmCo-based magnetic filler, because its Curietemperature allows operation of the sensor under relatively hightemperature conditions. Another embodiment utilises lower costNdFeB-based magnetic filler when such high temperatures are notrequired.

[0023] Alternative ways of applying a fixed bias field include the useof magnetic or superconducting materials, magnetic coatings or cores,and solenoid systems. Another way of applying the bias field is to makeintrinsic use of the magnetic properties of the sensing element. Forexample at the core of the amorphous or nanocrystalline wires there is adifferent domain configuration from the shell of the wire which couldprovide an intrinsic biasing field which varies in amplitude along theelement. An example of such a material would be Fe_(69.5)Cr₄Si_(7.5)B₁₅to which FIG. 5 relates.

[0024] However, it is also possible to apply a bias field where thepattern can be varied. For example the pattern could be of a fixedfunctional form (e.g. sine wave, sawtooth, etc.) which is altered (forexample swept) in amplitude (i.e. the pattern shape is retained butaltered in magnitude—a temporal variation), and/or location along thesensor element (a spatial variation). Alternatively or additionally thepattern itself could be changed, i.e. a change in pattern other thanmerely by change in position along the element. Such changes includevariations in the actual shape and/or changes in the average level ofthe amplitude, i.e. the addition of a spatially constant bias offset—forexample the addition of a spatially constant bias to the field of FIG. 4will enable the sensitive location c to be moved along the sensorelement.

[0025] Variations in the bias field pattern can be effected for exampleby the appropriate use of solenoids. Changes in bias pattern can be usedto facilitate the resolution of (magnetic field or) stress componentsalong the sensor element. As mentioned above, when using a ramped biaspattern, the addition of a spatially constant bias pattern can producemovement of a sensitive location. A similar effect could be obtained bysweeping the existing pattern along the element, while altering theamplitude of the ramp, i.e. its slope, will control the length of thesensitive location. It will be understood that normally the rate of anychange in the bias field needs to be low relative to the rate ofmeasurement (c.f. Tyren above, where the applied field changes at a ratemuch greater than the rate of measurement).

[0026] Furthermore, by selectively coating regions of the sensor elementwith more highly conductive coatings, these regions are effectivelyshort circuited (there is only a surface current at the frequenciesemployed), and play a much reduced or negligible in the sensoroperation. This means that only the stress S (or magnetic field H forGMI sensors) existing at uncoated regions will be included in the signalintegration, thereby enabling the effect of certain undesirable localmagnetic fields, including components of biasing fields, to be maskedout as required.

[0027] From the foregoing considerations it should be clear that bysuitably constructing and/or controlling the sensor, it is possible tomeasure stress or magnetic field at one or more restricted locationsalong the length of the sensor. This location or these locations can bepredetermined, for example by a design feature such as a patternedconductive coating, or controllable, for example by variation of theapplied bias magnetic field. Such a property is useful where it isdesired to avoid the difficulties and expense of soldering togetherseveral distinct elements or sensors.

[0028] The material of the sensing element may be a cobalt richamorphous alloy, for example of Co_(72.5)Si_(12.5)B₁₅. Other alloyscontaining traces of Mn, Fe, C, Nb, Ni, Cu, Mo and Cr can also be used.Other compositions include Fe₈₁B_(13.5)Si_(3.5)C₂,Fe_(4.9)Co_(71.8)Nb_(0.8)Si_(7.5)B₁₅, Co₈₀B₂₀, Fe_(77.5)Si_(7.5)B₁₅,Ni₈₀Fe₂₀, Fe_(69.5)Cr₄Si_(7.5)B₁₅,(Co_(0.94)Fe_(0.06))_(72.5)Si_(12.5)B₁₅ and Fe_(73.5)Cu₁Si_(13.5)B₉.Cobalt rich amorphous or nanocrystalline alloys have extremely highmaximum tensile strength values typically of between 1 to 4 GPa, whichmeans that they are very suitable for use where a robust sensor isrequired. They also have a high elastic modulus typically of around 100GPa for a Co rich ribbon. In addition they exhibit high corrosionresistance.

[0029] Moreover, measurement on a CoSiB wire indicate that the sensorelements are generally insensitive to changes in temperature at least inthe range 20 to 150° C., making them suitable for use in environmentswhere significantly elevated temperatures are likely to be encountered.It will be appreciated that this is the case for measurements of stressin road tyres, inter alia.

[0030] The sensor element may be in the form of a wire, ribbon or fibreproduced for example by melt spinning. Wire and ribbons are typically 10to 125 microns thick (minimum dimension). In manufacture, quenchingnormally results from the melt spinning process, and residual stressesarising therefrom couple with the magnetostriction to hinder domainrotation and so reduce the GMI/GSI effect. It is therefore preferred toanneal the quenched product to increase the sensitivity of the sensor,for example by furnace annealing, pulse current annealing or directcurrent annealing.

[0031] The sensor may comprise a single sensor element. It is possibleto embed such a sensor comprising a sensor element, e.g. in the form ofa ribbon, wire or large (elongate) fibre within an electricallyrelatively insulating supporting matrix, and in such a case the sensorwill be responsive to stresses applied to or transmitted through thematrix. By way of example, coupling to the sensor element may beinductive, capacitive or via embedded conductors.

[0032] A typical example would comprise a ribbon or wire embedded in avehicle tyre for sensing stresses applied to the tyre when in use. It iscommonly recognised that the tyre to contact patch is the area where itis desirable to be able to instantly sense where the frictional forceavailable is approaching the lower limit necessary for traction.Knowledge of the stress-strain dynamics of the tyre close to the ground,coupled with a model of the dynamic behaviour of the vehicle in responseto the contact patch forces would provide an almost instantaneousdetection of the dangers associated with changes in the nature of theroad surface, etc., such as incipient skids and aquaplaning, and mightalso provide information on tyre wear. A SAW sensor for such anapplication has been proposed in European Patent Application No.99114450.2. However, it is considered that the elastic properties of therubber/steel matrix will have a significant effect on the acoustic wavepropagation, and render such sensing difficult to employ in practice.

[0033] Alternatively, the sensor may comprise a plurality of sensorelements, e.g. in the form of discrete wires or ribbons, or as fibres.Where the elements are sufficiently large, they may be coupled togetheras desired, for example two or more ribbons or wires in series toprovide a larger sensor element. Such coupling may be by any suitablemeans such as by direct electrical contact, or by coupling withnon-magnetic wires therebetween. Again the sensor elements may beembedded in an electrically relatively insulating supporting matrix ifdesired

[0034] Where the sensor elements are relatively small, such asrelatively short fibres, it may be preferable to support them in anelectrically relatively insulating supporting matrix. Where there is nodirect contact between the elements, the properties of the sensor arethen determined by the matrix as a whole, and the bias means may bearranged to apply the bias field to the whole matrix. Preferably theindividual elements have some degree of alignment along a preferredaxis, but the matrix should still work as a sensor element even wherethe alignment is substantially random. Ordering may be accomplished byany known means, for example by preferential orientation brought aboutby the process of extruding the matrix material, or by the applicationof a magnetic field. Such fibres may have added benefits in terms ofincreasing the mechanical strength of the supporting composite.

[0035] Where the sensor element is embedded in a matrix, or comprises amatrix, typical matrix materials therefor are plastics (syntheticresins) and rubbers. Commonly, these types of matrix material areelectrically insulating.

[0036] Nevertheless, it is possible to employ matrix material which havea degree of conductivity provided this is significantly less than thatof the magnetic sensor elements(s) at the electrical frequency of use.An insulating matrix material may be rendered electrically conductive bysuitable loading with a conductive material, e.g. in fine particulateform. In such cases while the GSI effect will modify the impedance ofthe embedded fibre/wire/ribbon, this will then modify the impedance ofthe matrix as a whole, which will be sensed.

[0037] There are a number of examples of the use of the changing fieldat a magnetic sensor to enable displacement to be measured. An exampleis disclosed in U.S. Pat. No. 4,119,911 (Johnson) in which a permalloysensor is used to measure field variations as magnets are moved todetect their motion. Permalloy is not normally a soft amorphous ornanocrystalline material, and does nor show GMI or GSI effects. Thepresent invention relates to application of varying bias fields tochange the intrinsic properties of amorphous or nanocrystalline GMI andGSI materials.

[0038] Further features and advantages of the invention will becomeapparent upon consideration of the appended claims, to which the readeris referred, and upon a reading of the following description of anexemplary embodiment of the invention made with reference to theaccompanying drawings, in which:

[0039]FIG. 1 shows in schematic form the characteristic variation inimpedance of a GSI element in the form of a wire of(Co_(0.94)Fe_(0.06))_(72.5)Si_(12.5)B₁₅ (10 cm in length and 125 μm indiameter.

[0040]FIG. 2 shows in schematic form the characteristic variation inimpedance of a GSI element in the form of a Co-based amorphous ribbon,20 mm long, 1 mm wide and 20 μm thick, the exact composition of which isunknown, under various levels of applied stress;

[0041]FIG. 3 illustrates in schematic form the desensitising effect of asawtooth bias field;

[0042]FIG. 4 illustrates in schematic form the use of a ramped biasfield to control the position where a sensor is sensitive;

[0043]FIG. 5 shows the impedance response for a 20 mm length of a ribbonmade from the material Fe_(69.5)Cr₄Si_(7.5)B₁₅ with a width of 1 mm anda thickness of 20 μm.

[0044]FIG. 6 schematically illustrates the field profile along thelength of one form of biasing element for use in the invention; and

[0045]FIG. 7 illustrates the results obtained according to oneembodiment of the invention in the form of a graph of variation ofimpedance level with applied magnetic field for different applied stresslevels.

[0046]FIG. 5 shows that the response is relatively field insensitivewithout an externally applied bias field. It also shows that forgenerally equal increments of stress of around 71.2. MPa thedifferential response drops progressively, so that there is very littledifference between responses shown in plots D and E. The plots A0 and A1represent measurements taken before and after the other measurements,respectively.

[0047] A rubber mat loaded with NdFeB magnetic powder was rolled into amulti-turn cylinder and subjected to a diametric uniform magnetic fieldpulse from a Hirst Magnetiser system to produce when the mat M isstraightened a magnetic intensity distribution (field profile) along themat length L (width W). NdFeB is neodymium iron boron, a permanentmagnetic material similar to SmCo. The resulting field profile F isschematically illustrated by the arrows in FIG. 6 for a two-turncylinder, i.e. 4 pole pitches, along a 25 mm long mat). The mat lengthwill in general be chosen to substantially match the length of theresistive element with which it is to be used, which can be anythingfrom a few microns to several metres, permitting either point orintegral measurement of stress.

[0048] The straightened NdFeB loaded mat is placed adjacent to a 20 mmlong FeCoSiB amorphous ribbon to produce a sensor according to theinvention, and FIG. 7 shows in schematic form the impedance thereof atvarious stress levels as a function of an externally applied magneticfield. Each horizontal plot relates to one stress level. It will be seenthat the presence of the biasing mat leads to a characteristic whichvaries with applied stress, but which is substantially independent ofthe level of the externally applied magnetic field.

[0049] While particular reference has been made to the measurement ofstress in a vehicle tyre, it will be appreciated that there are otherapplications of the sensor of this invention, including the measurementof stress in aircraft wings and machine parts, including moving parts,and in the monitoring of stress levels in structures where straymagnetic fields constitute a problem, such as bridges where fields maybe produced by moving vehicles. A pressure transducer, e.g. for usewithin a tyre, may comprise such a sensor.

1. A sensor comprising a generally elongate magnetically soft amorphous or nanocrystalline electrically resistive element and biasing means for applying to the element a bias magnetic field, the component of said field directed along the length of the sensor having a spatially substantially varying pattern of amplitude along the element.
 2. A sensor according to claim 1 wherein said pattern is predetermined.
 3. A sensor according to claim 1 wherein said biasing means is controllable to effect a change in said pattern.
 4. A sensor according to claim 3 wherein said change is in the amplitude of the pattern.
 5. A sensor according to claim 3 or claim 4 wherein said change is in the position of said pattern relative to the length of the element.
 6. A sensor according to claim 3 or claim 4 wherein said change is in the pattern itself.
 7. A sensor according to any preceding claim wherein said amplitude variation is periodic.
 8. A sensor according to any one of claims 1 to 6 wherein said pattern includes rising and/or descending substantially linear ramps, or approximations thereto.
 9. A sensor according to any preceding claim wherein said element has portions along its length coated with a relatively highly conducting material.
 10. A sensor according to any preceding claim wherein said element is in the form of a wire, ribbon or fibre.
 11. A sensor according to any preceding claim wherein said element is melt spun.
 12. A sensor according to claim 10 wherein said element is annealed after being melt spun.
 13. A sensor according to any preceding claim wherein said element exhibits a small negative magnetostriction.
 14. A sensor according to any one of claims 1 to 12 wherein said element exhibits a small positive magnetostriction.
 15. A sensor according to any preceding claim wherein the element is formed of a cobalt rich alloy.
 16. A sensor according to any preceding claim wherein the element is embedded in a matrix.
 17. A sensor according to any one of claims 1 to 15 wherein a plurality of said elements are embedded in a matrix.
 18. A sensor according to claim 16 or claim 17 wherein said matrix is electrically insulating.
 19. A sensing device comprising a sensor according to any preceding claim and current supply means for supplying an alternating current to the element.
 20. A device according to claim 19 wherein the alternating current frequency lies in the radio-frequency range.
 21. A device according to claim 20 wherein the alternating current frequency is at least 100 kHz.
 22. A device according to claim 21 wherein the alternating current frequency is at least 1 MHz.
 23. A sensing device according to any one of claims 19 to 22 wherein said current supply means is directly coupled to the sensor element.
 24. A sensing device according to any one of claims 19 to 22 wherein the current means is indirectly coupled to the element, such as by inductive or capacitative or rf coupling.
 25. A sensing arrangement comprising a device according to any one of claims 19 to 24 and measuring means for measuring the alternating voltage generated in the element by the alternating current.
 26. An arrangement according to claim 25 wherein the element forms part of a resonant circuit.
 27. An arrangement according to claim 25 wherein the element forms part of a bridge circuit.
 28. A method of reducing the sensitivity of the impedance of a generally elongate magnetically soft amorphous or nanocrystalline stress sensor element to external magnetic fields, the method comprising applying along the sensor element a bias magnetic field, the component of said field directed along the length of the sensor having a spatially substantially varying amplitude pattern along the element.
 29. A method of measuring stress in an object by securing thereto a stress sensor comprising a generally elongate magnetically soft amorphous or nanocrystalline stress sensor element, supplying an ac current to the element, and measuring the impedance of the element, wherein a bias magnetic field with an amplitude which varies with position along the element, the component of said field directed along the length of the sensor having a spatially substantially varying amplitude pattern along the element to reduce its sensitivity to external magnetic fields.
 30. A method according to claim 28 or claim 29 wherein said pattern is predetermined.
 31. A method according to claim 28 or claim 29 wherein said pattern is changed.
 32. A method according to claim 28 or claim 29 wherein the magnitude of said pattern is changed.
 33. A method according to claim 31 or claim 32 wherein the position of said pattern along the element is changed.
 34. A method according to claim 29, or any one of claims 30 to 33 as dependent upon claim 29, wherein the object is a tyre, or part of a pressure transducer, or a part of a bridge, or a part of an aircraft, or an aircraft wing.
 35. A tyre, r a pressure transducer within a tyre, having embedded therein a sensor according to any one of claims 1 to
 18. 36. A vehicle provided with a tyre according to claim 35 and having detecting means for detecting changes in the impedance of the sensor element.
 37. A vehicle according to claim 36 wherein the detecting means is secured to a fixed part of the vehicle.
 38. An aircraft having secured thereto a stress sensor according to any one of claims 1 to
 18. 39. An aircraft according to claim 38 wherein the sensor is secured to a wing.
 40. A bridge having secured thereto a stress sensor according to any one of claims 1 to
 18. 41. A method of sensing stress applied to a flexible matrix material comprising embedding a sensor according to any one of claims 1 to 15 in the matrix and measuring its impedance.
 42. A method according to claim 41 wherein the matrix material is the carcase of a tyre.
 43. A sensor element substantially as hereinbefore described with reference to FIGS. 6 and 7 of the accompanying drawings. 